Change Prefixes .foundicon-[icon] to .foundicon-[set]-[icon] in order to use more than 1 set in the same project.

Hi,
I had a problem : using bower, I installed foundation-icons and had to use 2 different sets on my project.
The problem was that general set and social set were conflicting using sass files:
The font family for my general icons were overwritten by social icons font.

So, I propose to add the set to change the default code to use icons from : `<i class="foundicon-[icon]"></i>` to `<i class="foundicon-[set]-[icon]"></i>`.

I used :  
- `general` for the general set
- `accessibility` for the accessibility set
- `enclosed` for the general enclosed set
- `social` for the social set.

I think it easily solve the problem for people using your fantastic tool.

Thanks for your interest,
Julien